1. Field of the Invention
The present invention relates to a process for the preparation of diene rubber vulcanizates and the vulcanized rubbers afforded by such a process.
2. Discussion of the Background
Diene rubbers are amongst the most frequently used all-purpose rubbers. The term diene rubbers is understood as meaning polymers and copolymers of butadiene, styrene and also isoprene. Natural rubber and synthetic polyisoprene are also to be regarded as diene rubbers. These rubbers are inexpensive and available in large amounts and have good general properties. A disadvantage of this group of rubbers is a structure-related limitation of inadequate aerobic and anaerobic ageing resistance and heat stability of their sulfur vulcanizates.
Anaerobic ageing is understood as meaning the behavior of the vulcanizate with the exclusion of air, including, for example, in the heating mold, where prolonged reaction times cause so-called reversion in many diene rubbers. This understood as meaning the decrease in important material properties, such as tensile strength, elongation at break, modulus and dynamic properties when the mixture is heated for longer than the optimum period of time. The phenomenon of reversion can be observed most simply with the aid of a so-called vulcameter curve in accordance with DIN 53 529.
Aerobic ageing, on the other hand, includes all processes of natural ageing in the presence of atmospheric oxygen, including at elevated temperatures, such as, for example, on vehicle tires under operating conditions.
The deficiencies described in aerobic and anaerobic ageing are particularly pronounced in natural rubber and polyisoprene.
In order to compensate, at least partly, for this disadvantage, in addition to using antioxidants, a number of alternative vulcanization recipes have been used. These include so-called sulfur donors, such as tetramethylthiuram disulfide (TMTD), dimorpholine disulfide (DTDM) and similar compounds, or so-called EV systems which use the sulfur employed more efficiently by employing a larger amount of accelerator at a lower sulfur dosage. With these vulcanization systems, however, the processing reliability is often impaired because of the shorter partial vulcanization time. The vulcanizates produced with EV systems also have various disadvantages, including, in particular, their increased susceptibility to fatigue under continuous dynamic stress. When TMTD and generally low molecular weight dithiocarbamates are employed, there is moreover the risk of the formation of a considerable amount of carcinogenic nitrosamines. Above all, however, the improvements which can be achieved in ageing stability are not yet convincing, but still mainly leave something to be desired.
Another way of improving the heat stability of diene rubber vulcanizates has been proposed in EP-OS 0 385 072 and 0 432 417. In this procedure, compounds which contain the customary radicals which accelerate sulfur vulcanization and have the general structure ##STR2## where R=ethyl and n=2 (EP-OS 0 385 072), called BDTE below, and where R=benzyl and n=2 (EP-OS 0 432 417) called BDBzTE below,
and where n=6, called BDBzTH below (EP-OS 0 432 417) are employed as the sole crosslinking agents. PA1 a) BDTE is susceptible to the formation of hazardous nitrosamines. PA1 b) The vulcanization reaction proceeds considerably more slowly with the crosslinking agents BDBzTE and BDBzTH (t.sub.90 values of 29 minutes and 34.3 minutes), so that uneconomically long vulcanization times of 40 minutes or more are required at 150.degree. C. PA1 c) Because of the high molecular weight of BDBzTE and BDBzTH, at least 7 parts must be employed per 100 parts of rubber in order to achieve useful vulcanizate properties, which represents an economic burden on the process according to EP-OS 0 385 072 and 0 432 472. PA1 Where such large amounts of chemicals are used, there is furthermore the risk that the secondary products formed in the course of the vulcanization are no longer adequately soluble in the elastomer, but lead to undesirable efflorescence phenomena. PA1 1. A vulcanization rate similar to conventional sulfur/accelerator systems. PA1 2. A crosslinking yield, measured by the vulcanizate properties such as tensile strength, modulus and compression set, similar to conventional sulfur/accelerator systems. PA1 3. A high resistance to reversion, that is to say anaerobic ageing resistance. PA1 4. An outstanding aerobic ageing resistance of the vulcanizates. PA1 5. A reduction in the amount of crosslinking agent used for technical reasons (risk of blooming) and also for economic reasons. PA1 n=6 (BDBzTH),
The examples demonstrate that, for example, vulcanizates having a very good ageing stability are obtained with 4.5 parts of BDTE or 7 parts of BDBzTE per 100 parts of rubber. BDBzTH with a longer network bridge is also used as a comparison example (EP-OS 0 432 417, Table 1a, Examples 1a and Da). However, the disadvantages of the system are also not to be left unmentioned:
In both the patent documents mentioned, attention is drawn expressly to the adverse effect generally of sulfur additions in respect of the stability of the vulcanizate to reversion.
The above-mentioned European patent applications thus provide no indication at all as to how diene rubber vulcanizates of outstanding ageing stability can be obtained with economically and industrially acceptable amounts of crosslinker substances over acceptable vulcanization times.
The two DE-PSS 22 65 382 and 22 56 511 relate very generally to the compounds of the general formula EQU A--S--S--R--S--S--A' (b)
wherein the radicals A and A' represent a large number of accelerator radicals, inter alia including N-substituted thiocarbamoyl radicals, and R represents almost any divalent organic radical. This results in an exceptionally wide selection of substances, of which some are employed in the examples. These also include an analogous substance according to the general formula (a) where R=methyl and n=2, called BDMTE below. In DE-PS 22 65 382, this is also employed together with sulfur (column 31/32, Table VII), but the experiments show that even small amounts of sulfur (0.3 to 1%) lead to undesirable reversion phenomena. It is expressly admitted that batches without addition of sulfur have a higher capacity for resistance to reversion (column 32, lines 1-11).
BDMTE and BDTE are employed as crosslinker substances in DE-PS 22 56 511. This specification refers to the insufficient crosslinking density of the vulcanizates as a disadvantage (column 21, lines 12 to 27), which can be compensated by additions of 0.5-1.5 parts of sulfur, although the penalty again is an increased susceptibility of the rubber mixtures to reversion.
Generally, a criticism of the prior art last mentioned is that the absence of reversion phenomena in the rheometer test is used as the sole criterion of heat stability. However, as stated above, rheometer testing at best provides some information on anaerobic ageing--but even this is only incomplete, since the rheometer curve merely provides information on the torque of the vulcanizate. No data are given in DE-PSS 22 65 382 and 22 56 511 on the aerobic ageing resistance, which gives much more relevant indications of the resistance of the vulcanizate when used in practice. This information is preferably obtained by storage of vulcanizate samples in a circulating air cabinet under continuous exposure to a temperature of 100.degree. C. over a period of 3, 7 or 14 days (in accordance with DIN 53 508). After this time has elapsed, which represents exceptionally severe exposure conditions for natural rubber, cis-polyisoprene, cis-polybutadiene and blends thereof, all the relevant elastomer properties are measured, such as tensile strength, elongation at break, modulus, hardness and rebound resilience, and if appropriate also the dynamic properties and the abrasion resistance. A comprehensive insight into the resistance of the elastomer to thermal ageing is thus obtained.
Thus, there remains a need for a process for obtaining, under appropriate conditions, heat- and ageing-stable vulcanizates of diene rubbers under practical use conditions.